1. Field of the Invention
The present invention relates to a polyvinyl alcohol film which has uniform optical characteristic over large area and has large width, the polyvinyl alcohol film being suitable for use of a polarization film used in a liquid crystal display having large screen.
2. Description of the Prior Art
A polarizer having functions of transmission and shielding of light is a fundamental element of a liquid crystal display (LCD), together with a liquid crystal having a function of switching light. Applied fields of such an LCD have expanded from small apparatuses such as electronic calculators, wristwatches and the like in early days of its development, to, in the recent years, apparatuses in a wider range such as lap top personal computers, word processors, liquid crystal color projectors, navigation systems for automobiles, liquid crystal televisions, personal phones, indoor and outdoor measuring apparatuses and the like, which have relatively large display area, and there is a need for polarizers having uniform optical characteristic or performance over larger area than conventional products.
For obtaining a polarizer having general construction, a polyvinyl alcohol film (hereinafter, polyvinyl alcohol-based polymer is abbreviated as a xe2x80x9cPVAxe2x80x9d, and a polyvinyl alcohol-based (polymer) film is abbreviated as a xe2x80x9cPVA filmxe2x80x9d, in some cases) is monoaxially stretched and dyed to produce a polarization film on both faces of which protective films such as a triacetic acid cellulose and the like are laminated. For obtaining uniformity of polarization characteristic, there are may important requirements such as use of a PVA film having uniform thickness, uniform dyeing of a dichroic dye, laminating without irregularities, and the like, and degree of uniformity of a PVA film which is a material used for the polarization film is an important factor.
As a method of producing a PVA film, there is industrially employed, for example, a method of casting on belt or drum in which a film material, which may contain an organic solvent and the like, containing PVA in the form of a solution or melted substance is fed onto a belt or drum heated, and dried to form a film.
In a method in which a film material containing PVA in the form of a solution or melted substance is discharged onto the belt or the drum and dried to form a film, it is difficult to obtain a film having uniform thickness. Naturally, thickness irregularities of a film are desired to be small, and the best choice is to have almost none of thickness irregularities, however, there are actually several problems. Namely, thickness irregularities have two items: large waviness containing unevenness in thickness over an area of a length within several cm to some tens cm along the TD direction (transverse direction), and local streaks caused by thickness irregularities occurring on a film over an area of a length within 1 mm.
The target of the present invention is concerned with the latter local streaks, wherein linear streaks (unevenness) may occur continuously along the MD direction (longitudinal direction) from a discharging portion or lip of a die with the lapse of time in discharging a film material from the die. Though this streak was not conventionally recognized as a problem, a problem has been found that with recent increase in screen size and increase in screen luminance of LCD, when a PVA film has streaks, a polarization film produced from this film shows color irregularity, leading to an optical defect. Conventionally, trials to solve thickness irregularities of a film have been made widely, and for example, Japanese Laid-open Patent Publication No. 6-138319 suggests a PVA film having small thickness irregularities. However, this suggestion has an object to solve large waviness containing unevenness in thickness over an area of a length within several cm to some tens cm along the TD direction, and substantially, no technologies directed to solve a local streak defect have been known to date.
On the other hand, with recent increase in screen size of LCD, optical films having a width of at least 2 m are required, however, for producing a film having a width of at least 2 m, it is necessary to connect belts along the MD direction in the case of the method of casting on belt. As a consequence, when a PVA film material is discharged onto a belt and dried, the product can not be used as an optical film in some cases, due to optical insufficiencies (ununiformity of refractive index, transmittance, crystallinity and the like) caused by local streaks on the connecting portion of belts.
An object of the present invention is to provide a PVA film which has uniform optical characteristic over in large area and has large width, the PVA film being suitable for use of a polarization film utilized in a liquid crystal display having large screen. The present inventors have intensively studied for attaining this object, and devised the present invention.
The above-mentioned object is accomplished by a PVA film wherein the film is obtained by casting on drum, and the variance in thickness along the TD direction of the film is 0.5 xcexcm/mm or less, the thickness of the film is within the range of 20 to 150 xcexcm, and the width of the film is at least 2 m. Here, xe2x80x9cthe variance in thickness along the TD direction of the film is 0.5 xcexcm/mm or lessxe2x80x9d means a fact that when the thickness of a film is measured continuously along the TD direction (transverse direction defined in JIS K 6900) of a film and difference in the thickness of a film per 1 mm in length of any portion along the TD direction, this difference in thickness is 0.5 xcexcm or less. According to the present invention, a PVA film having uniform optical characteristics over a large area is obtained.